An insulation resistance test typically uses an applied DC voltage to measure the insulation resistance of the conductors of an electrical supply network. Such insulation tests are, for example, necessary to avoid fires caused by faulty wires. Test procedures for insulation tests are defined in various standards, including the ANSI/NETA ATS-2009 standard. This standard provides test procedures and acceptances levels for most types of electrical equipment. According to ANSI/NETA ATS-2009, for electrical supply networks with a nominal voltage of 250V, the minimum test voltage has to be 500V for the insulation test. For different nominal voltages of electrical supply networks, higher or lower test voltages are required.
The insulation resistance for an electrical supply network with a nominal voltage of 250V must be no lower than 500 kΩ. For testing the insulation resistance, the test voltage has to be applied to each pair of conductors of the electrical supply network, e.g., to the phase conductor and the neutral conductor, to the neutral conductor and the protective earth conductor, and to the protective earth conductor and the phase conductor, in a single-phase alternating current electrical supply network. None of these three measured resistances are allowed to be lower than 500 kΩ to pass the insulation resistance test. In most cases, the measured resistances exceed the critical value of 500 kΩ significantly.
The insulation resistance test measurements have to be performed in predetermined time intervals or after modifying the electrical supply networks. Before starting an insulation test, all appliances connected to the electrical supply network must be disconnected. Due to the high test voltage, devices like computers, telephones, or televisions can be destroyed by the test voltage. In addition, appliances connected to an electrical supply network during insulation testing can distort the test results.
In many cases, in particular in offices, an electrician that performs the test may find it difficult to identify all of the appliances connected to the electrical supply network to be tested. Some appliances may be covered by desks, boards, or the like, and are therefore not visible. As a result, in some cases, even if mandatory, the insulation test is not performed because electricians are concerned about destroying appliances. This raises the risk of fires or electric shocks.
It is an objective of the present disclosure to overcome the problem of identifying when appliances are connected to electrical supply networks by providing a technique for testing whether appliances are still connected before performing an insulation test. The present disclosure also provides, in various embodiments, an apparatus and a method for improving insulation testing.